A wireless local area network (wireless LAN) has become widespread as a wireless communication system. Carrier sense multiple access collision avoidance (CSMA/CA) is often used in a wireless LAN.
In a system that uses CSMA/CA, a transmission terminal checks whether a wireless resource is being used by other terminals before the transmission terminal starts transmitting data. When the wireless resource is not being used by other terminals, the transmission terminal transmits a data signal. This procedure permits an avoidance of a packet collision. However, a packet collision may occur when a plurality of terminals start transmitting data at the same time or when there exists a hidden terminal. When a packet collision has occurred, a reception terminal is not successful in receiving data.
When the data reception is not successful, a retransmission control is performed between the transmission terminal and the reception terminal. In general, the retransmission control is performed by, for example, a hybrid automatic repeat request (HARQ). The HARQ can perform a retransmission control with incremental redundancy.
In a wireless communication system that uses a retransmission control with incremental redundancy, a portion of encoded bits are transmitted in a first transmission, as illustrated in FIG. 1A. The encoded bits include data and a parity bit. The parity bit is redundancy information used to decode the data. Data and a portion of the parity bit are transmitted in the first transmission. In the example illustrated in FIG. 1A, data and a parity bit P1 are transmitted in the first transmission.
The reception terminal tries to decode data using received bits. Here, as illustrated in FIG. 1B, the reception terminal tries to decode the data using the parity bit P1. When the data is decoded properly, retransmission is not performed. On the other hand, when the data is not properly decoded, retransmission is performed.
A parity bit that has not been transmitted yet is transmitted in a retransmission scheme (that is, in the second and subsequent transmissions). The parity bit transmitted in the second and subsequent transmission may hereinafter be referred to as a “redundancy bit”. For example, when the first transmission is not successful, a parity bit P2 illustrated in FIG. 1A is transmitted as a redundancy bit. Then, the reception terminal tries to decode the data using previously received bits and a newly received redundancy bit. Here, the reception terminal tries to decode the data using the parity bits P1 and P2, as illustrated in FIG. 1B. Likewise, a retransmission control is performed repeatedly until data is decoded properly. Here, parity bits P3, P4, . . . are sequentially transmitted as a redundancy bit.
A technology that applies HARQ to a wireless LAN protocol is disclosed in, for example, Japanese Patent No. 5254369. Further, a retransmission control with incremental redundancy is disclosed in, for example, J. F. Cheng et al., “Adaptive Incremental Redundancy,” IEEE 58th Veh. Technol. Conf. (VTC 2003-Fall), vol. 2, pp. 737-741, October 2003.
As described above, a wireless communication system that uses a retransmission control with incremental redundancy has been proposed. However, with respect to a conventional retransmission control with incremental redundancy, the number of iterations of an iterative decoding that affects a transmission latency is not taken into consideration, so there is room for improvement in the efficiency in retransmission. Further, for example, with respect to a retransmission control used in a conventional wireless LAN, there is a need for a source terminal to obtain a right to transmission again when a destination terminal is not successful in receiving data. However, the source terminal is not always able to immediately obtain a right to transmission during a next time period (contention window: CW) in which a right to transmission is to be obtained, so there is a possibility that a transmission efficiency (throughput) will be decreased because it may take a long time to perform a retransmission control.